Failure Diagnosis

Part of Gear Making

Reading worn and broken gear teeth to identify root causes and prevent recurring failures.

Why This Matters

A gear that has failed contains information. The pattern of wear or fracture tells you why it failed — overloading, misalignment, lubrication failure, material defect, or manufacturing error. Reading this evidence accurately means you can make a better replacement gear, not just a copy of the failed one. Without diagnosis, you will rebuild the same failure mode repeatedly.

Gear failure diagnosis is a skill developed through observation and systematic thinking. It does not require instruments beyond a magnifying glass and a steel rule. The key is knowing what patterns to look for and what each pattern means. This knowledge saves materials and labor by directing you to the correct fix rather than guessing.

In a rebuilding context where re-cutting gears is expensive in time and skill, understanding failure modes also helps you design conservatively enough that first failures are avoidable, or at minimum, are the least damaging type.

Wear Patterns and What They Mean

Normal wear — a smooth, polished appearance on the working tooth flank, symmetrical on both sides of the pitch line — indicates the gear is functioning as designed but may be approaching end of life from material loss. Normal wear is acceptable and expected. Action: monitor tooth thickness; replace when tooth thickness at pitch circle has reduced by more than 20–25%.

Abrasive wear — rough, scratched, matte surface appearance on tooth flanks, often with directional scratch marks — indicates abrasive particles in the lubricant. Sources: sand or grit contamination, wear particles from running-in, corroded surfaces breaking off. Action: improve sealing, change lubricant, clean the gear housing thoroughly, investigate the source of abrasive.

Adhesive wear (scuffing/galling) — local metal transfer between tooth surfaces, rough irregular patches where metal has cold-welded and then torn away — indicates the oil film broke down and metal-to-metal contact occurred. Usually a result of overloading, under-lubrication, or excessive speed. The contact temperature became high enough to break down the lubricant. Action: reduce load or speed, upgrade lubricant (higher viscosity or EP — extreme pressure — additive), improve cooling.

Pitting — small rounded craters on the tooth flank surface, concentrated near the pitch line — indicates contact fatigue. Repeated rolling contact develops subsurface microcracks that eventually break out as surface pits. Initial (shallow, smooth-edged) pitting may stabilize with normal wear. Progressive (deepening, spalling) pitting will destroy the tooth surface. Action: reduce load, use harder or tougher material for replacement gear, improve lubrication.

Fracture Patterns and What They Mean

Root fillet fracture — crack initiating at the tooth root fillet (the curved junction between tooth and root circle), propagating across the full tooth section leading to complete tooth loss — is the classic overload failure. The root is the highest-stress location in a tooth under bending. Root fracture through the full tooth means the bending stress exceeded the material’s fatigue strength.

Identifying root fracture: the fracture surface is relatively flat, often showing beach marks (concentric curved lines indicating crack propagation front position over multiple loading cycles) if it was a fatigue failure, or a rough, granular surface if it was a single overload.

Fatigue root fracture: Beach marks visible; the tooth likely held for many thousands of load cycles before failing. Indicates a stress concentration or material defect, or a marginally undersized tooth. Fix: larger module (bigger teeth), better root fillet radius, higher-quality material.

Overload root fracture: No beach marks; rough granular surface; usually a single-cycle failure from an impact or sudden overload. Fix: understand the overload event (jam, shock load, foreign object). Add a torque-limiting clutch or shear pin to protect from future overloads.

Tip fracture — chips or cracks at the tooth tip rather than the root — indicates interference (the tip of one gear digging into the dedendum of the other) or foreign object damage. Interference occurs when the tooth counts are too low or the center distance is too small. Fix: increase tooth count (especially on the smaller gear), verify center distance, check for interference in design.

Bearing and Shaft Failures Associated with Gears

Gear failures are often preceded or accompanied by bearing and shaft failures:

Fretting at gear/shaft interface: If the gear is a loose fit on its shaft, small sliding motion under torque causes fretting corrosion — a reddish-brown powder and damage to both surfaces. Fix: tighten the fit, add a key, or use a properly designed interference fit.

Shaft fatigue fracture: If the gear was misaligned, the shaft experiences bending every revolution — fatigue fracture of the shaft at the gear location. Typically the shaft fractures at the key slot or at a section change. Fix: improve alignment, round off sharp section changes (stress concentrations).

Bearing failure patterns: Race spalling (pitting of the bearing race) indicates overload or contaminant; spinning inner race indicates inadequate fit; false brinelling (evenly spaced indentations at rolling element spacing) indicates vibration while stationary.

Systematic Diagnosis Procedure

1. Record the failure history. How long did the gear run before failing? What changed before the failure (load increase, lubrication change, repair to adjacent components)?

2. Examine the failed gear. Identify the failure mode (wear pattern, fracture type) from the descriptions above.

3. Examine the mating gear. Often shows complementary damage that helps confirm the diagnosis.

4. Check the bearing condition. Worn or failed bearings allow shaft movement that damages gears.

5. Check alignment. Evidence of edge-loaded wear (concentration of wear pattern at one end of the tooth face) indicates misalignment.

6. Check lubrication. Was lubricant present at time of failure? Is the feed system working? Is the lubricant the correct type?

7. Identify the root cause from the accumulated evidence.

8. Fix the root cause first, then replace the damaged gear.